In recent years, porous membranes such as ultrafiltration membranes and micro-filtration membranes are used in various fields such as recovery of electrodeposition paint, removal of microparticles from ultrapure water, production of pyrogen-free water, concentration of enzymes, sterile filtration and clarification of fermentation liquid, and treatment of clean water, sewage, and drainage. In particular, porous hollow fiber membranes have high packing density per unit volume of the membrane and make it possible to miniaturize a treatment device, thereby being widely used.
In the case of filtrating various fluid to be treated using porous hollow fiber membranes, there is a serious problem that adsorption, blockage, or deposition of a part of inorganic substances and/or organic substances that are contained in the fluid to be treated occurring inside the small membrane pores or on the membrane surface, so-called fouling, decreases the water permeability performance.
As a method of controlling such a fouling phenomenon, a physical washing method (so-called air scrubbing) to introduce air into a receptacle of hollow fiber membranes to vibrate liquid inside the receptacle so as to remove microparticles adhering to the surfaces of the hollow fiber membranes is disclosed in Patent Document 1. Not only for the casing type modules as described in Patent Document 1 but also for, for example, non-casing type (immersion type) modules to be often used in MBR (a membrane bioreactor) and the like, a method of controlling fouling by introducing air from the bottom of the module is generally used. However, this method has a problem that, although it is possible to effectively inhibit fouling of the membranes, a phenomenon of the membranes coming into contact with each other to block small pores on the membrane outer surface, so-called “fretting”, tends to develop and, as a result, the water permeability performance of the membranes decreases in a long-term operation.
To enhance the effect of this air scrubbing, a membrane with a creative shape is also disclosed. In Patent Document 2, a membrane is disclosed in which hollow fibers are provided with serpentine-shaped crimps so as to inhibit a decrease in treatment performance resulting from hollow fibers coming into contact with each other to lower the membrane surface area and from retention of fluid.
In Patent Document 3, a membrane is disclosed in which minute protrusions are provided at a part of the peripheries of the hollow fiber membranes so as to enhance the effect of air scrubbing. In Patent Document 4, a membrane of the same shape as that in Patent Document 3 to be used in dialysis is disclosed.